/* 
 *   @author:- MAHMUDUL HASAN SAKIB
 *   DATE & TIME :- 2024-08-13 19:55:46
 *   BANGLADESH , SYLHET.
 */ 

#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <deque>
#include <forward_list>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <optional>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
#define fi first
#define se second
#define endl '\n'
#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define vi vector<ll>
#define maxn 500005
#define mod 1000000007
#define inf 1000000007
#define pii pair<ll,ll>
#define vii vector<pii>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define lcm(a,b) ((a*b)/__gcd(a,b));
#define srt(v) sort(v.begin(),v.end())
#define rsrt(v) sort(v.rbegin(),v.rend())
#define rep(i, a, b) for(int i = (a); i < (b); i++)
#define per(i, a, b) for(int i = (a); i > (b); i--)
#define rev_str(str) reverse(str.begin(),str.end());
#define print(v) for(auto e:v) cout<<e<<" "; cout<<endl;
#define printp(v) for(auto e:v) cout<<e.first<<" "<<e.second<<endl;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
ll dp[100005];

void solve() {
        ll n,m;cin>>n>>m;
        vector<string>p(n);
        rep(i,0,n) cin>>p[i];
        while (m-- && sz(p) > 1) {
            ll j= 0;
            string max1 = p[0]+p[1];
            rep(i,1,sz(p)-1){
                string s = p[i]+p[i+1];
                if(s>=max1){
                    max1=s;
                    j=i;
                }
            }
            p[j] = max1;
            p.erase(p.begin()+j+1);
        }
        string max_ = *max_element(all(p));
        cout << max_ << endl;
}

int main() {
    fast;
    ll t = 1;cin >> t;
    while (t--) {
        solve();
    }
    return 0;
}